Increasing bus speed in an electronic system is a fundamental hurdle in higher performance system design. The system bus often limits overall system performance since the bus provides the means of communication between system components. Many factors complicate the task of increasing the frequency of switching the bus.
In general, signal distortion caused by various transmission line effects such as ringing and wave reflection increases when switching frequency increases. A malfunction can ultimately occur when the distortion is large enough to cause an erroneous voltage level to be sampled. Thus, improved measures are required to compensate for these increasing transmission line effects.
Wave reflections generally occur at discontinuities in signal lines and are often the primary source of signal distortion. Stubs or branches in the bus, device connections, and physical ends of buses are all examples of discontinuities in the bus. Consequently, a reconfiguration which adds devices, removes devices, or changes stub lengths alters the wave reflections in a system. This increases the difficulty of reconfiguring the system.
Modern computer systems often need to be scalable and upgradeable, making reconfiguration difficulties highly undesirable. A scalable computer system allows enhancement of computing power by adding components. For example, a second, third, or fourth microprocessor may be added to a scalable system to increase processing throughput. An upgradeable computer system may allow the addition of components or may allow removal and replacement of a particular component such as a microprocessor. Such system enhancement may be unduly limited in computer systems which are difficult to reconfigure.
Bus terminations limit wave reflections in some prior art systems. One prior art scheme is addressed in "A Dynamic Line-Termination Circuit for Multireceiver Nets", IEEE Journal on Solid-State Circuits, Vol. 28, No. 12, December 1993, pages 1370-1373. In this reference, the receiver terminates a line to incoming signal values. This scheme essentially implements a keeper formed by an input buffer and an active termination device which immediately drives values back onto the line. The keeper thus latches values as they are received on the signal line. Process variations and changing operating conditions can reduce the effectiveness of this scheme by causing keeper impedance to drift and no longer match the line impedance.
In this prior art scheme, the switching of the active termination device during an incident voltage wave can produce a step voltage on the signal line. In a typical application, many such dynamic termination circuits are used on a signal line, and each termination circuit will contribute to signal line noise with its step voltage, resulting in the incident voltage wave being altered by each of the termination circuits. These noise-causing step voltages occur at different times since the receivers are generally located at different distances from the source of the wave. Additionally, once all of the receivers have switched and are terminating the signal line to its present value, the next time a driver tries to drive the opposite value on the signal line, it will have to overcome all of the active termination devices. This may result in poor performance in a system with many receivers, especially since the termination device impedance is as low as the line impedance.
Thus, the prior art termination circuit does not terminate using an impedance greater than the bus impedance, nor does it provide a termination which is coupled to selectably terminate the bus to a termination voltage. As a result, the bus is continuously terminated by all receivers at the bus impedance. Such termination can detriment system switching performance.
Other prior art systems terminate ends of a bus with discrete termination resistors. Often, these termination resistors add to system cost by requiring longer circuit board routing, additional stubs, and/or additional circuit board layers. The resistors, which may be individually packaged or integrated into a bus termination chip, also increase computer system costs. Extra stubs added to accommodate the termination resistors generally complicate the bus network and increase system noise (such as wave reflection).
As wave reflections increase, end terminations may not provide sufficient system noise control. This may be true even if a termination is provided on the same circuit card as the bus driver. Consider an off-chip termination for an output driver on an integrated circuit chip in a typical package. A first conductor typically couples the driver on the chip to a connection on a circuit board. Another conductor couples the termination to the chip. Finally, the chip is coupled to the system bus. As a result of this three way connection, the noise generated at the chip reaches both the system bus and the termination.
Minor system reconfiguration can adversely affect a high speed computer system with end terminations because any new or different wave reflections caused by these changes propagate all the way to the bus end before being terminated. This may be problematic in a scalable or upgradeable computer system which requires the ability to reconfigure by adding or changing bus components. Additionally, a system designer may wish to alter a particular stub length if the system needs redesign or improvement.
Resistive end terminations are generally continuously enabled and terminate the bus with a resistance matching the characteristic impedance of the bus. A low characteristic impedance bus advantageously allows a given driver to provide high current through a signal line, providing fast switching for a given load. Unfortunately, when a matched termination is used, low impedance translates to a large current flow through the termination. Thus, efforts to conserve power are hampered when such matched terminations are used and bus impedance is lowered to boost performance.
Accordingly, numerous problems afflict prior art terminated computer system buses. System reconfiguration and enhancement are difficult in some systems. Other systems generate noise in their attempts to reduce it, consume excessive amounts of power, or fluctuate in effectiveness due to processing and operating conditions. System cost is often increased by the addition of termination components, and many termination systems are simply becoming inadequate as bus frequencies increase.
Prior art resistive termination circuits generally do not terminate using an impedance greater than the bus impedance. Nor are the terminations distributed throughout the system and coupled to selectably terminate to a termination voltage.